Add validation for feature-flags configmap to webhook

[lbernick]

Prior to this commit, the validating admission webhook validated the logging, defaults, and leader election configmaps, but not the feature flags configmap. This meant that invalid edits to the feature-flags configmap would be silently accepted, but could later cause PipelineRuns and TaskRuns to fail. This commit validates the feature-flags configmap in the validation webhook.

Note: we currently have a cyclic dependency between the validating admission webhook and the configmaps used by pipelines. This commit does not address this problem, which should be handled separately (#4542).

Tested locally.

/kind bug

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Release Notes

Added validation for feature-flags configmap

[JeromeJu]

/assign

[wlynch]

/lgtm

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[lbernick]

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